I. Field of the Invention
The present invention relates generally to DC-DC power converters and, more particularly, to such a converter which utilizes a transmission line as the energy storage device.
II. Description of Related Art
There are many previously known DC-DC power converters which utilize an energy storage device coupled with a switch to boost the voltage output to a load. Such power converters are also known as boost converters. Although different devices may be employed to store the charge necessary to boost the output voltage to the load, many of these previously known converters utilize an inductor for storing the charge to boost the voltage output.
There are, however, previously known power converters which utilize a transmission line as the charge storing device used to boost the output voltage. Unlike an inductor where all the energy storage is lumped into a single element, energy in a transmission line is stored as waves which travel between the ends of the transmission line. The distributed nature of this type of storage means is that the same transmission line can be used to simultaneously store the charge from two or more separate power converters and then use switch timing to multiplex these signals.
This same property also means that the fraction of the line inductance devoted to each converter can be changed as the energy demands shift simply by changing the switch timing.
In a normal inductor based circuit, the necessary inductance will be set by the maximum energy storage that will be needed by the power converter. During the periods when the converter is not operated at its peak capacity, the excess inductance is not used. However, with a transmission line converter, the inductance can simply be redistributed to another power converter, thus lowering the overall total needed inductance for the power converters, as well as lowering the volume, weight, and cost of the converters.
At switching frequencies up to tens of megahertz where low power resistance power switches are currently available, the necessary transmission line length is very long, i.e. on the order of inches to meters. In order to make a compact transmission line converter able to operate at these frequencies, previously known converters have utilized a circuit known as a Rayleigh network as a lumped element simulation of the transmission line.
In a Rayleigh network, the network includes a plurality of inductors of the same value mounted in series with each other. In between each pair of inductors, a capacitor connects the junction between the two inductors to ground. The capacitors are also all of the same value.
Although the Rayleigh network does approximate the transmission characteristics of a transmission line at high frequencies as the number of different segments, i.e. each inductor/capacitor pair, rises, it does so slowly. For example, an ideal transmission line has an infinite number of resonances. These resonances are identified as poles or zeros on an impedance versus frequency graph where the impedance goes to either zero or infinity to produce the infinity or zero, respectively.
In a lumped network, each additional LC (inductor/capacitor) segment has a resonance thus bringing the simulation of the transmission line closer to an ideal transmission line.
However, with a Rayleigh network, even the low frequency poles and zeros do not converge to the correct frequencies without a high number of LC segments. Indeed, in order to produce an acceptable transmission line power converter, 40 or more LC segments are required to simulate a transmission with acceptable accuracy. If a fewer number of LC segments are employed, the Rayleigh network exhibits poor conversions of the poles and zeros. This in turn generates unnecessary losses for the Rayleigh network. Furthermore, in the event that multiple converters are used with the same transmission line, a Rayleigh network with fewer than about 40 LC segments will exhibit cross talk between the converters.
The disadvantage of the Rayleigh network is that, because such a large number of discrete capacitors and inductors are required to simulate a transmission line with acceptable accuracy, the Rayleigh network requires a large amount of printed circuit footprint, cost in view of the large number of components, and increased labor cost for assembly.